1. Field of the Invention
The present invention relates to a semiconductor device, a manufacturing method of the semiconductor device, a liquid crystal display device, an RFID (Radio Frequency Identification) tag, a light emitting device, and an electronic device. In particular, the present invention relates to a semiconductor device in which characteristics of the semiconductor device are improved by thinning a gate insulating film and a leak current can be reduced, a manufacturing method of the semiconductor device, a liquid crystal display device, an RFID tag, a light emitting device, and an electronic device.
2. Description of the Related Art
When a semiconductor device is miniaturized, a thickness of a gate insulating film is thinned by scaling law. In a conventional semiconductor device formed over a single crystal silicon substrate, a tunnel current is generated when a gate insulating film is thin. In order to overcome an increase of the tunnel current, an insulating film having a high dielectric constant such as a hafnium oxide film or an aluminum oxide film tends to be employed for a gate insulating film so as to have a thin equivalent oxide thickness while keeping a substantial thickness of the gate insulating film. A tunnel current is particularly generated in a thickness of 2 nm or less.
However, when an insulating film having a high dielectric constant is used for a gate insulating film, interface state density cannot be reduced. Therefore, there is proposed a method by which interface state density is reduced by forming a silicon nitride film in an interface with a single crystal silicon substrate, that is, a method by which interface state density is reduced by using a film formed of an aluminum oxide film stacked over a silicon nitride film for a gate insulating film.
More in detail, an amorphous silicon oxide film is formed over a surface of a single crystal silicon substrate, and an aluminum film is formed over the amorphous silicon oxide film. Thereafter, by heat treatment, an aluminum oxide film is formed over the single crystal silicon substrate. Then, by a microwave low temperature plasma nitriding method, a silicon nitride film is formed between the surface of the single crystal silicon substrate and the aluminum oxide film. Accordingly, a gate insulating film that is a stacked film of the silicon nitride film and the aluminum oxide film is formed over the single crystal silicon substrate (see Reference 1: Japanese Patent Application Laid-Open No: 2005-86023).
In a semiconductor device formed over a glass substrate, as shown in FIG. 28A, a base insulating film 11 including two layers is formed over a glass substrate 10, and an amorphous silicon film 12 is formed over the base insulating film 11. Thereafter, as shown in FIG. 28B, the amorphous silicon film 12 is irradiated with laser light to form a crystalline silicon film 13, and a gate insulating film (not illustrated) formed of a silicon oxide film or the like may be formed over the surface of the crystalline silicon film 13. The surface of the crystalline silicon crystallized by a laser has the surface roughness (unevenness on the surface) that is significantly large as compared with single crystal silicon. Therefore, a current leak path such as a tunnel current tends to be generated in a portion with a thin thickness of a gate insulating film, which is formed of a silicon oxide film (or a silicon oxynitride film) having a thickness of approximately 5 nm by a plasma CVD (chemical vapor deposition) method or a plasma oxidizing method. The current leak path is easily generated when a silicon oxide film becomes thin, which is formed on the top or in a concave portion of unevenness on the crystalline silicon film surface shown in FIG. 28B. In other words, when a gate insulating film is thinned, a tunnel current due to unevenness on a crystalline silicon film surface is increased.
In addition, in the conventional semiconductor device formed over the single crystal silicon substrate as described above, a stacked film in which an aluminum oxide film is stacked over a silicon nitride film is used as a gate insulating film; therefore, the silicon nitride film is in contact with the silicon surface. In this structure, transistor characteristics such as a threshold value or a hysteresis are lowered as compared with a structure in which a silicon oxide film is in contact with a silicon surface. In other words, in consideration of the transistor characteristics, it is desirable that a silicon oxide film be in contact with a silicon surface.